The present invention relates to the production of concentrated sulphuric acid and oleum from feed gases with up to 70% SO2+SO3 (SOx) and a content of H2O corresponding to H2O/SO3 molar ratios of up to 1.6, particularly in the range of 0.5 to 1.6 upstream an intermediate sulphuric acid condenser. The feed gases are produced by combustion of sulphur and sulphur compounds and feed gases from wet scrubbing of SO2 gases originating from roasting of metal sulphides or from thermal regeneration of spent sulphuric acid and sulphates as well as feed gas produced from combustion of flue gases rich in H2S, such as flue gases with 90 vol % H2S. Up to 99.95% of the SOx in the feed gas can be recovered as typically 98.5-99.5 wt % concentrated sulphuric acid and/or oleum with up to 25 wt % SO3. Furthermore, the process of the invention is concerned with minimizing the power consumption of the sulphuric acid plant, minimizing the consumption of cooling water and obtaining maximum possible recovery of the heat liberated in the process for production of high pressure steam for power production. It is a further concern of the process of the invention to avoid corrosion by hot sulphuric acid at any concentrations by i.a. using air cooled glass tubes in the intermediate and final sulphuric acid condensing stages of the process.
It has been known for many years to produce concentrated sulphuric acid from strong SO2-gases containing up to 50 vol % SO2 with SO2-conversions of up to 99.9% or more by two-step catalytic SO2-conversion with intermediate absorption of SO3 or condensation of H2SO4 in both an intermediate and in a final absorption or condensation steps. In principle, SO3 in the gas phase is transferred to the liquid phase by absorption of the SO3 in the liquid phase, while H2SO4 vapour is transferred to liquid phase by condensation in which the gas is cooled to below its H2SO4 dew point either by direct contact with circulating acid used as the coolant, or in falling film condensers in which the gas is cooled to below its dew point and the acid is condensed on surfaces of air cooled glass tubes. In known processes, except the one described in our U.S. Pat. No. 7,361,326, both absorption or condensation steps take place in packed towers or other types of scrubbers cooled by circulating sulphuric acid being cooled by cooling water in acid coolers. The large amount of heat liberated in the absorption or condensation towers is usually lost to cooling water.
Some patents such as U.S. Pat. No. 5,130,112 describe how to utilize some or all of the heat of cooling the circulating acid for production of low pressure steam or heating of water but such heat exchangers must be made of highly acid resistant alloys which are expensive and only corrosion resistant when operated below 220-240° C. and with acid strengths above 98.5-99 wt % H2SO4.
German patent DE 19522927 B4 describes a process in which a gas with H2O/SO3 molar ratio of 0.9-1.1 is cooled in an intermediate condenser from above its H2SO4 dew point to a temperature below 160° C. in a heat exchanger in which the gas and the condensate are cooled by flowing downwards across bundles of boiler tubes carrying boiler feed water and/or boiling water passing upwards in the tube bundles counter current to the gas and condensate. The applicability of this system is strongly limited by availability of sufficiently acid resistant alloys for the boiler tubes.
Our U.S. Pat. No. 7,361,326 discloses a double condensation process for production of concentrated sulphuric acid from feed gases with up to 30% SO2 and H2O/SO2 ratio above about 1. In the first step of the process, most of the SO2 is converted to SO3 where after the gas is passed to an intermediate condenser in which SO3 and H2SO4 vapour is condensed as concentrated sulphuric acid either in a packed tower cooled by circulating acid or in vertical, air cooled glass tubes with either up flow or downflow of the gas in the tubes. The latter is mentioned as an option to avoid flooding at high gas velocities but is said to convey the disadvantage that it produces sulphuric acid of low concentration (70-85 wt %), thus requiring a subsequent concentration stage, such as a packed tower to reach the desired sulphuric acid concentration of 98 wt % or above. The exit gas from the intermediate condenser passes through a second SO2 conversion step and subsequently to a final wet condensing stage under the addition of particles. This patent is not concerned with utilisation of the heat released in the intermediate condenser, which in all the examples of the patent is transferred to circulating sulphuric acid and lost to cooling water.
It is therefore an object of the present invention to overcome the disadvantages of prior art processes, in particular to provide an improved double condensation process for production of highly concentrated sulphuric acid with up to 99.95% SO2-conversion and with improved utilization of the heat released in the intermediate condenser, more particularly with recovery of up to 96% of all heat released in the process for production of super heated high pressure steam for power production with minimal risk of corrosion of the heat exchangers and the sulphuric acid condensers.
These and other objects are solved by the invention.